Various configurations and methods are known in the art to remove acid gas from a process gas (e.g., various distillation-, adsorption- and absorption processes), and among those regenerator-absorber systems are frequently employed as a relatively robust and cost-efficient gas purification system.
In a typical regenerator-absorber system, gas is contacted in an absorber in a counter-current fashion and the acid gas (or other gaseous component) is at least partially absorbed by a lean solvent to produce a rich solvent and a purified process gas. The rich solvent is then typically heated in a cross heat exchanger and subsequently stripped at low pressure in a regenerator. The so stripped solvent (ie., lean solvent) is cooled in the cross heat exchanger to reduce the temperature in the lean solvent before completing the loop back to the absorber. Thus, such regenerator-absorber systems typically allow continuous operation at relatively low cost. However, in many circumstances the extent of the acid gas removal efficiency is not satisfactory, and especially where the acid gas is carbon dioxide, stringent emission standards can often not be achieved with a standard regenerator-absorber system.
To overcome problems associated with carbon dioxide removal in such systems, the temperature in the regenerator may be increased. However, increased corrosivity and solvent degradation often limit the degree of optimization for this process. Alternatively, a split-flow absorption cycle may be employed in which the bulk of the solvent is removed from an intermediate stage of the regenerator column and recycled to an intermediate stage of the absorber. A typical split-flow process is described by Shoeld in U.S. Pat. No. 1,971,798. In this arrangement only a small portion of the solvent is stripped to the lowest concentration, and a high vapor to liquid ratio for stripping is achieved in the bottom trays of the regenerator, resulting in somewhat lower energy use at relatively low outlet concentrations. However, the reduction in energy consumption is relatively low due to thermodynamic inefficiencies in stripping (mainly because of variations in the solvent composition as it circulates within the split loop).
To circumvent at least some of the problems with the split loop process, various improvements have been made. For example, one improvement to the split-flow process is to more accurately control the concentration of solvents. To more accurately control the solvent concentrations, two modifications are generally necessary. The first modification comprises an intermediate reboiler, which may be installed to a main regenerator to boil off water from the semi-lean solvent to adjust the concentration of the semi-lean solvent stream to the concentration of the lean solvent. The second modification comprises a side-regenerator to regenerate condensate from the main regenerator. The condensate from the main regenerator is sent to the top section of the main regenerator, where it undergoes partial stripping, and is then further stripped to a very low concentration of dissolved gas in the side-regenerator, before being returned to the bottom reboiler of the main regenerator.
Since only a relatively small portion of the total solvent (typically ˜20%) is stripped to the ultra-low concentration, relatively low outlet concentrations with comparably low energy use may be achieved. Furthermore, when methyl diethanolamine (MDEA) is employed as a solvent in the improved split-flow process, the liquid circulation can be reduced by about 20%. However, the modifications to improve energy use and lower solvent circulation generally require a substantial modification in the configuration of the main regenerator, and the installation of a side-regenerator, both of which may result in substantial costs and significant down-time of an existing absorber-regenerator system.
Another improvement to the split-flow process is described by Camell et al. in U.S. Pat. No. 6,139,605. Here, two regenerator columns are utilized wherein a primary regenerator produces a semi-lean solvent and wherein a secondary regenerator produces an ultra-lean solvent. A small portion of the purified process gas leaving the absorber is expanded to a lower pressure level thereby producing a cooled purified process gas. The heated ultra-lean solvent stream leaving the secondary regenerator is cooled by the cooled purified process gas thereby producing a heated purified process gas, which is subsequently fed into the secondary regenerator. The recycled gas is then recovered from the secondary regenerator and fed back into the feed gas stream at the absorber.
The use of a heated process gas instead of a reboiled solvent at the secondary regenerator advantageously lowers the partial pressure of the solvent vapor in the secondary regenerator, and allows the secondary regenerator to operate a lower temperature than the primary regenerator column. Operating the secondary regenerator at a reduced temperature typically results in a reduced corrosivity of the solvent, which in turn may allow for the use of cheaper materials such as carbon steel in place of the conventional stainless steel. Furthermore, a split-flow process using vapor substitution may be combined with fixed-bed irreversible absorption technology, e.g. to remove H2S and or COS from the recycle gas in a bed of solid sorbent, thereby ensuring a relatively long bed life of the absorber. However, due to the use of recycle gas and the use of a secondary regenerator column, retrofitting of existing absorber-regenerator combinations may be relatively expensive and time consuming.
Therefore, although various improvements to the basic configuration of an absorber-regenerator process are known in the art, all or almost all of them suffer from one or more disadvantages. Therefore, there is a need to provide improved configurations and methods for the removal of a gaseous component from process gases.